Periscopic beam directors have generally been designed as all-reflective systems, for applications that must handle a wide spectral range. Example such applications include a beam-director mounted on the surface of a host vehicle. In such applications, the beam director typically is mounted proximate a cutout in the frame of the host vehicle.
An optical dome is mounted over the cutout, so as to allow the beam director to extend beyond the cutout, so that the beam director can view its field of regard. The optical dome also operates to protect the optics, and to maintain host vehicle pressure seal (e.g., for high altitude deployment).
In order to fold the light around within the outer dome, periscopic beam directors generally require a dome with an inner diameter at least 4 to 5 times the diameter of the clear aperture (sometimes referred to as free aperture or objective aperture) of the optical beam. As is known, the clear aperture is the opening in the mount of an optical system that restricts the extent of the rays incident on the given surface. The clear aperture is typically circular and specified by its diameter.
What is needed, therefore, is a periscopic beam director with a hemispheric field of regard, in a volume that minimizes external structure or dome size when mounted on a host vehicle.